High Energy Stereoscopic System

Dark Matter Search In The Inner Galactic Halo: An Update With Ten Years Of Data

March 2016

Fig. 1:
Projected dark matter density of a Milky Way-like galaxy from the
cosmological N-body simulation Aquarius. The brightness
of the image is proportional to the logarithm of the squared dark
matter density along the line of sight. (Image credit: Aquarius
project.)

It is a widely accepted idea that dark matter, which contributes about
27% to today’s total energy content of the universe, consists of
elementary particles. So far, dark matter agglomerations have been
detected only very indirectly due to the impact of their mass on the
dynamics of the surroundings. One example is the fact that the
observed velocity of stars in some galaxies is too large to keep them
bound to the galaxy by gravity, if only the mass of visible matter is
accounted for.

Since dark matter particles have not been directly detected so far,
they must be electrically neutral, stable (or have a lifetime
exceeding the age of the universe) and non-baryonic. Furthermore,
observations of the matter distribution in the universe and
simulations of their structure formation tell us that a large portion
of the dark matter must be in the form of massive,
i.e. non-relativistic, particles, termed cold dark matter. Among the
well-motivated elementary particle candidates for cold dark matter are
so-called weakly interacting massive particles (WIMPs); these are
(quasi-)stable particles which are supposed to interact only via the
weak interaction, with masses and coupling strengths at the
electroweak scale. If produced in a standard thermal history of the
Universe, such WIMPs would have a relic density matching well today's
observed dark-matter density. Their inferred velocity-weighted
annihilation cross section – a measure for the probability that two
dark-matter particles close to each other would annihilate – is about
3x10-26 cm3s-1. This cross section is
often referred to as the "natural scale" to probe for their existence
in indirect dark-matter searches. In these searches, one looks for the
presence of stable standard model particles produced from the
annihilation process, e.g. high-energy gamma rays or neutrinos.

The number of gamma rays observable from dark-matter annihilations
does not only depend on the annihilation cross section, but also on
the square of the number density of the dark-matter
particles. Therefore, regions in the sky that have a putatively large
dark matter density are most promising targets for dark matter
searches. As suggested by simulations, this is particularly true for
the centers of galaxies where the largest dark matter density is
expected (Fig. 1). It is for this reason that the central region of
our own galaxy (the Galactic Centre, GC) is a prime target for dark
matter searches. Furthermore, since the gamma-ray intensity decreases
with increasing target distance, the GC has the particular advantage
of being located close to Earth (at a distance of 8.5 kpc).

Fig. 2:
Gamma-ray significance map of the inner 300 parsecs seen by
H.E.S.S. The seven annuli used for the dark matter search are
indicated by the blue solid circles. The region of the sky
excluded from the data analysis, containing the astrophysical
gamma-ray sources HESS J1745-290, G0.9+01 and the diffuse
emission, is shown by the red box.

The inner 300 parsecs region of the Milky Way has been observed by the
H.E.S.S. telescopes since 2004, making it one of the regions with the
largest exposure to search for very-high-energy (VHE, E > 100 GeV)
emission. Several VHE gamma-ray sources have been discovered in the GC
region, such as HESS J1745-290 spatially coincident with the
supermassive black hole Sagittarius A*, the pulsar wind nebula inside
the supernova remnant G0.9+0.1, and a band of diffuse emission along
the Galactic plane. Therefore, while dark matter signals from the GC
are expected to be larger than those from nearby galaxies by orders of
magnitude, searches for these signals in the GC region face a strong
standard astrophysical background (as opposed to other targets such as
dwarf galaxies which are essentially free of these backgrounds).

A 10-year monitoring during the years 2004-2014 of the GC with the
H.E.S.S. phase-1 array results accumulated a total exposure time of
254 hours. This is more than twice the exposure used in a previous
H.E.S.S. search for dark-matter annihilations in that region. Much like the previous search, the
analysis presented here makes use of a circular search region with a
radius of 1° centered on the Galactic Center (Fig. 2). However,
contrary to the previous search, the sensitivity of this work is
improved by taking full advantage of the fact that both the energy and
spatial distribution of the expected gamma-ray emission due to dark
matter annihilation in the search region - which for instance is
expected to be strongly peaked towards the center - are much different
from any expected background emission. To this end, the search region
is subdivided into concentric annuli of of 0.1° width; for each of
these, the expected gamma-ray emission due to dark matter annihilation
is compared to the data for different assumptions on the annihilation
cross section, the dark matter particle mass and the physics of the
annihilation process. The region along the Galactic plane dominated by
standard astrophysical emission (red rectangle in Fig. 2) is excluded
from the analysis.

The data analysis does not show any significant gamma-ray excess above
the expected background in any of the annuli. Therefore, constraints
on the DM velocity weighted annihilation cross section can
be derived, shown in Fig. 3. Limits are calculated as a function of
the (unknown) dark-matter particle mass ranging from 200 GeV to 70
TeV, assuming that annihilation takes place into W+W- or τ+τ-
pairs, respectively. A standard Einasto profile is assumed for the
dark matter density distribution. The constraints derived in this
analysis improve over the previous analysis by a factor 5 (in the W+W-
channel), reaching = 6x10-26 cm2s-1 at a dark matter mass
of 1 TeV (Fig. 3 top). In the τ+τ- channel, the
H.E.S.S. measurements are able to probe the thermal cross section for
dark matter particles between 400 GeV and 2.5 TeV (Fig. 3 bottom).

Fig. 3:
Constraints on the velocity-weighed annihilation cross section
for the W+W- (upper panel) and τ+τ- (bottom panel)
channels derived from 10 years of observations of the inner 300 pc
of the GC region with H.E.S.S. The constraints are expressed as
95% CL upper limits as a function of the dark matter mass mDM. The
observed limit is shown as black solid line. The expected limits
are calculated from 1000 Poisson realizations of the background
measured in blank-field observations at high Galactic
latitudes. The mean expected limit (black dotted line) together
with the 68% CL (green band) and 95% CL (yellow band) containment
bands are shown. The blue solid line corresponds to the limits
derived in a previous analysis of 4 years (112 h of live time) of
GC observations. The horizontal black long-dashed line corresponds
to the thermal relic velocity-weighted annihilation cross section.